Hulking three-row SUVs are a product of America, a huge shiny mirror that reflects all of our excesses and immodesties. Several classic big utes—the Cadillac Escalade, the Chevrolet Suburban, and the Lincoln Navigator among them—are, fittingly, produced by hometown carmakers. But foreign companies have been studying the art of the luxury hauler, and Mercedes-Benz’s GLS-class proves that those efforts are paying off. The GLS-class is a stylish, spacious, and dynamically sound take on...